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Formwork Design & Hyperstatic Reactions

Formwork Design & Hyperstatic Reactions

(OP)
Had an interesting query from the formwork/shoring engineers today.

They have a note saying that their formwork has not been designed to take loads imposed by PT. So he has requested a hyperstatic loading plan.

However... I think that the loads imposed by the PT will be minimal because at the time of stressing (~7 days) the structural connection between the slab and column will be much more stiff compared to the adjacent props and thus take the majority of the hyperstatic load.

Has anyone else produced a hyperstatic loading plan or encountered a similar request?

RE: Formwork Design & Hyperstatic Reactions


Very familiar topic (I think), but first please define what is meant by "hyperstatic".

Wikipedia says: "statically indeterminate (or hyperstatic) when the static equilibrium equations are insufficient for determining the internal forces and reactions on that structure."
Webster's says nothing.
Collins says: "(of a building) having excessive or redundant support components."

If this is a 2-way banded tendon layout in a flat slab, then I can understand wanting to know more about the load redistribution that occurs during post-tensioning operations. It can be a very real problem for the shoring if the system has not anticipated loads.

Post-tensioning sequence also has an influence on how the loads are redistributed.

Perhaps a better description of your structure would help to determine what the formwork/shoring engineer is looking for.

Ralph
Structures Consulting
Northeast USA

RE: Formwork Design & Hyperstatic Reactions

I haven't provided a plan but I do ask that the formwork and shoring folks consider the loads imposed by the PT. And, of course, I cooperate with that effort within reason. It can be tough to provide design guidance early on in the life of the project because, where issues arise, they are often associated with sequencing and constructability issues that aren't necessarily sorted at EOR design time. Some examples:

1) In a banded / distributed scenario, one might decide to tension the distributed tendons first for some reason. That would result in sort of a PT line load applied to the formwork over the unstressed banded tendons.

2) Weird things can happen at oddball pour strip situations etc.

I agree though, where sequencing is conventional and areas of reverse tendon curvature are kept close to vertical support, the PT shouldn't be working the shoring too hard.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Formwork Design & Hyperstatic Reactions


When you see shores under a beam exhibiting an "S" shape after slab post-tensioning has been completed (and no beam tendons were pulled), you know that the shoring IS being worked too hard.

Ralph
Structures Consulting
Northeast USA

RE: Formwork Design & Hyperstatic Reactions

(OP)
Two way flat plate (200mm thick). I have given the advice to stress the column strip tendons before the middle strip tendons.

Not quite sure how else the PT could cause buckling of the props.

RE: Formwork Design & Hyperstatic Reactions

Trenno,

agreed, the support strip/beam tendons should be at least partially stressed before the tendons loading that direction is stressed. The designer should be specifying the tendon stressing sequence, not just for the formwork but to avoid over stressing parts of the slab.

Unfortunately none of the design software will do this for the designer so they will have to understand what they are doing and work it out for themselves!

But getting them to do it on site without direct supervision might be more problematic.

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